Additive manufacturing system and method for printing three-dimensional parts using velocimetry

ABSTRACT

An additive manufacturing system that retains a print head for printing a three-dimensional part in a layer-by-layer manner using an additive manufacturing technique, where the retained print head is configured to receive a consumable material, melt the consumable material, and extrude the molten material. The system also includes a velocimetry assembly configured to determine flow rates of the molten material, and a controller assembly configured to manage the extrusion of the molten material from the print head, and to receive signals from the velocimetry assembly relating to the determined flow rates.

BACKGROUND

The present disclosure relates to additive manufacturing systems forprinting or otherwise building three-dimensional (3D) parts withlayer-based, additive manufacturing techniques. In particular, thepresent disclosure relates to flow rate detection techniques for use inextrusion-based additive manufacturing systems.

Additive manufacturing systems are used to print or otherwise build 3Dparts from digital representations of the 3D parts (e.g., AMF and STLformat files) using one or more additive manufacturing techniques.Examples of commercially available additive manufacturing techniquesinclude extrusion-based techniques, jetting, selective laser sintering,powder/binder jetting, electron-beam melting, and stereolithographicprocesses. For each of these techniques, the digital representation ofthe 3D part is initially sliced into multiple horizontal layers. Foreach sliced layer, a tool path is then generated, which providesinstructions for the particular additive manufacturing system to printthe given layer.

For example, in an extrusion-based additive manufacturing system, a 3Dpart may be printed from a digital representation of the 3D part in alayer-by-layer manner by extruding a flowable part material. The partmaterial is extruded through an extrusion tip carried by a print head ofthe system, and is deposited as a sequence of roads on a substrate in anx-y plane. The extruded part material fuses to previously deposited partmaterial, and solidifies upon a drop in temperature. The position of theprint head relative to the substrate is then incremented along a z-axis(perpendicular to the x-y plane), and the process is then repeated toform a 3D part resembling the digital representation.

In fabricating 3D parts by depositing layers of a part material,supporting layers or structures are typically built underneathoverhanging portions or in cavities of 3D parts under construction,which are not supported by the part material itself. A support structuremay be built utilizing the same deposition techniques by which the partmaterial is deposited. The host computer generates additional geometryacting as a support structure for the overhanging or free-space segmentsof the 3D part being formed. Support material is then deposited from asecond nozzle pursuant to the generated geometry during the printingprocess. The support material adheres to the part material duringfabrication, and is removable from the completed 3D part when theprinting process is complete.

SUMMARY

An aspect of the present disclosure is directed to an additivemanufacturing system that includes a print head for printing a 3D partin a layer-by-layer manner using an additive manufacturing technique.The print head is configured to receive a consumable material, melt theconsumable material, and extrude the molten material. The system alsoincludes a velocimetry assembly configured to determine flow rates ofthe molten material, and a controller assembly configured to manage theextrusion of the molten material from the print head, and to receivesignals from the velocimetry assembly relating to the determined flowrates.

Another aspect of the present disclosure is directed to a method forusing an additive manufacturing system, which includes providing a printhead retained by the additive manufacturing system, feeding a consumablematerial to the print head, melting the consumable material in the printhead to produce a pre-extrudate of the molten consumable material, andextruding the pre-extrudate from the print head as an extrudate. Themethod also includes determining flow rates of the pre-extrudate or theextrudate with a velocimetry assembly.

Another aspect of the present disclosure is directed to a method forusing an additive manufacturing system, which includes extruding amolten material from a print head retained by the additive manufacturingsystem as an extrudate, and routing a pulsed laser beam toward theextrudate to scatter light rays of the pulsed laser beam from theextrudate. The method also includes detecting at least a portion of thescattered light rays with a detector over multiple captured frames,wherein each captured frame has a speckle pattern, and comparing thespeckle pattern of the multiple captured frames to determine flow ratesof the extrudate.

DEFINITIONS

Unless otherwise specified, the following terms as used herein have themeanings provided below:

The term “extrudate” refers to a molten or partially molten materialafter exiting an extrusion nozzle. In comparison, the term“pre-extrudate” refers to a molten or partially molten material prior toexiting an extrusion nozzle, such as a molten or partially moltenmaterial flowing through the extrusion nozzle. Upon, exiting theextrusion nozzle, the “pre-extrudate” forms the “extrudate”.

The term “flow rate”, with reference to an extrudate or a pre-extrudate,refers to a velocity of the extrudate or pre-extrudate, a volumetricflow rate of the extrudate or pre-extrudate, or both.

The term “controller assembly”, with reference to an additivemanufacturing system, refers to one or more control circuits, one ormore computer-based systems, or combinations thereof, which areconfigured to manage the operation of the additive manufacturing system,and which may be internal and/or external to the additive manufacturingsystem.

The terms “preferred” and “preferably” refer to embodiments of theinvention that may afford certain benefits, under certain circumstances.However, other embodiments may also be preferred, under the same orother circumstances. Furthermore, the recitation of one or morepreferred embodiments does not imply that other embodiments are notuseful, and is not intended to exclude other embodiments from the scopeof the present disclosure.

Directional orientations such as “above”, “below”, “top”, “bottom”, andthe like are made with reference to a layer-printing direction of a 3Dpart. In the embodiments shown below, the layer-printing direction isthe upward direction along the vertical z-axis. In these embodiments,the terms “above”, “below”, “top”, “bottom”, and the like are based onthe vertical z-axis. However, in embodiments in which the layers of 3Dparts are printed along a different axis, such as along a horizontalx-axis or y-axis, the terms “above”, “below”, “top”, “bottom”, and thelike are relative to the given axis.

The term “providing”, such as for “providing a print head”, when recitedin the claims, is not intended to require any particular delivery orreceipt of the provided item. Rather, the term “providing” is merelyused to recite items that will be referred to in subsequent elements ofthe claim(s), for purposes of clarity and ease of readability.

The terms “about” and “substantially” are used herein with respect tomeasurable values and ranges due to expected variations known to thoseskilled in the art (e.g., limitations and variabilities inmeasurements).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top, front perspective view of an additive manufacturingsystem in use with consumable assemblies, which includes one or morevelocimetry assemblies.

FIG. 2A is a perspective view of a print head and guide tube for usewith the additive manufacturing system.

FIG. 2B is an exploded perspective view of the print head, showing apump assembly.

FIG. 3 is a perspective view of the pump assembly in use with the guidetube, a consumable filament, an extrudate velocimetry assembly, and afilament-feed velocimetry assembly.

FIG. 4 is a sectional view of section 4-4 taken in FIG. 3, illustratinga liquefier assembly of the pump assembly in use with the extrudatevelocimetry assembly.

FIG. 5 is an expanded view of a nozzle of the liquefier assembly in usewith the extrudate velocimetry assembly.

FIG. 6 is a schematic illustration of a detector of the extrudatevelocimetry assembly and processed speckle patterns over successiveframes.

FIGS. 7 and 8 are example logic diagrams for processing the specklepatterns over successive frames.

FIG. 9 is a flow diagram of an example method for determining anextrudate velocity from the processed speckle patterns.

FIG. 10 is an expanded view of the nozzle of the liquefier assembly inuse with a first alternative extrudate velocimetry assembly, whichutilizes a transmission-forward-scatter pattern.

FIG. 11 is an expanded view of the nozzle of the liquefier assembly inuse with a second alternative extrudate velocimetry assembly, whichutilizes a transmission-forward-scatter pattern, and which includesphotodetector pixels for measuring extrudate widths.

FIG. 12 is an expanded view of an alternative nozzle of the liquefierassembly in use with an alternative pre-extrudate velocimetry assembly,which utilizes a transmission-forward-scatter pattern.

DETAILED DESCRIPTION

The present disclosure is directed to an additive manufacturing systemand method for printing 3D parts and support structures usingvelocimetry. As discussed below, the additive manufacturing system mayinclude a velocimetry assembly (or multiple velocimetry assemblies)configured to determine material flow rates (e.g., velocities and/orvolumetric flow rates) of a print head extrudate and/or pre-extrudate,as defined above. The velocimetry assembly preferably transmits signalsrelating to the determined flow rates to a controller assembly of theadditive manufacturing system (e.g., a move compiler), which may thenutilize the received information to compensate for variations in thematerial flow rates.

For example, in a first embodiment, the velocimetry assembly may be usedas a calibration tool for each print head of the additive manufacturingsystem. In this embodiment, the velocimetry assembly may measure flowrates of the print head extrudate in response to different filamentdrive commands. This calibration routine may also be performedperiodically to identify gradual changes in the extrudate flow rates.For example, over extended periods of operation, the print heads maypotentially develop liquefier scaling, material accumulation, and thelike, which can change the relationships between the filament drivecommands and the resulting extrudate flow rates. Periodic calibrationsmay allow the controller assembly to compensate for these changes and/ormay be used to identify when a given print head should be cleaned orreplaced.

As a calibration tool, the velocimetry assembly is preferably located ata fixed location, such as at a purge station, allowing it to be usedwhen needed. Alternatively, the velocimetry assembly may be located at astand-alone station outside of the additive manufacturing system. Inthis case, each print head may be calibrated with the velocimetryassembly before being loaded or installed to the additive manufacturingsystem, and the calibration information may be transmitted to thecontroller assembly of the system for subsequent use.

In a second embodiment, the velocimetry assembly may be used duringprinting operations to monitor extrudate flow rates while printing 3Dparts and support structures. This can assist the controller assembly inmanaging the printing operations. Additionally, in this embodiment, thevelocimetry assembly may be used to identify extrusion variations, suchas tip clogging, extrudate back-pressure changes, and thermaldegradation of the consumable material, which can otherwise affectprinting operations. For example, thermal degradation of the consumablematerial can result in the material accumulating on the inner surface ofa print head liquefier, which can change (i.e., increase) response timedelays. When operating in this manner, a velocimetry assembly ispreferably retained by each print head or by a head carriage for theprint heads.

In a further embodiment, an additional velocimetry assembly may be usedto measure feed rates of consumable filaments as the filaments are fedto a print head. This may assist in further reducing response times, aswell as for detecting potential filament feed issues, such as filamentbreakage, jamming, and the like.

FIG. 1 shows system 10 in use with two consumable assemblies 12, whichillustrates a suitable additive manufacturing system that includes oneor more velocimetry assemblies for determining flow rates of print headextrudates (or pre-extrudates), and/or feed rates of consumablefilaments. Each consumable assembly 12 is an easily loadable, removable,and replaceable container device that retains a supply of a consumablefilament for printing with system 10. Typically, one of the consumableassemblies 12 contains a part material filament (“part materialconsumable assembly”), and the other consumable assembly 12 contains asupport material filament (“support material consumable assembly”).However, both consumable assemblies 12 may be identical in structure.

In the shown embodiment, each consumable assembly 12 includes containerportion 14, guide tube 16, and print heads 18. Container portion 14 mayretain a spool or coil of a consumable filament, such as discussed inMannella et al., U.S. patent application Ser. Nos. 13/334,910 and13/334,921. Guide tube 16 interconnects container portion 14 and printhead 18, where a drive mechanism of print head 18 (or of system 10)draws successive segments of the consumable filament from containerportion 14, through guide tube 16, to a liquefier assembly of the printhead 18.

In this embodiment, guide tube 16 and print head 18 are subcomponents ofconsumable assembly 12, and may be interchanged to and from system 10with each consumable assembly 12. In alternative embodiments, guide tube16 and/or print head 18 may be components of system 10, rather thansubcomponents of consumable assemblies 12.

System 10 is an additive manufacturing system for printing 3D parts ormodels and corresponding support structures (e.g., 3D part 20 andsupport structure 22) from the part and support material filaments,respectively, of consumable assemblies 12, using a layer-based, additivemanufacturing technique, and with the use of one or more velocimetryassemblies (not shown in FIG. 1). Suitable additive manufacturingsystems for system 10 include extrusion-based systems developed byStratasys, Inc., Eden Prairie, Minn. under the trademarks “FDM” and“FUSED DEPOSITION MODELING”.

As shown, system 10 optionally includes one or more purge stations(e.g., purge station 24, shown with hidden lines), and further includessystem casing 26, two bays 28, chamber 30, platen 32, platen gantry 34,head carriage 36, head gantry 38, z-axis motor 40, and a pair of x-ymotors 42. Purge station 24 is a suitable device for performing purgeoperations, where each print head may extrude a strand of the part orsupport material into a purge bucket, optionally followed by a tip wipeoperation, such as discussed in Turley et al., U.S. Pat. No. 7,744,364.As discussed above, in the first embodiment, the velocimetry assemblymay retained at purge station 24 to measure extrudate flow rates fromeach print head 18 during purge operations (e.g., for calibrationpurposes).

System casing 26 is a structural component of system 10 and may includemultiple structural sub-components such as support frames, housingwalls, and the like. In the shown embodiment, system casing 26 definesthe dimensions of bays 28, and of chamber 30. Bays 28 are container baysconfigured to respectively receive container portions 14 of consumableassemblies 12. Typically, each of bays 28 may be intended to receiveeither a part material consumable assembly 12 or a support materialconsumable assembly 12.

In an alternative embodiment, bays 28 may be omitted to reduce theoverall footprint of system 10. In this embodiment, container portions14 may stand adjacent to system casing 26, while providing sufficientranges of movement for guide tubes 16 and print heads 18. Bays 28,however, provide convenient locations for loading consumable assemblies12.

Chamber 30 is an enclosed environment that contains platen 32 forprinting 3D part 22 and support structure 24. Chamber 30 may be heated(e.g., with circulating heated air) to reduce the rate at which the partand support materials solidify after being extruded and deposited (e.g.,to reduce distortions and curling). In alternative embodiments, chamber30 may be omitted and/or replaced with different types of buildenvironments. For example, 3D part 22 and support structure 24 may bebuilt in a build environment that is open to ambient conditions or maybe enclosed with alternative structures (e.g., flexible curtains).

Platen 32 is a platform on which 3D part 20 and support structure 22 areprinted in a layer-by-layer manner, and is supported by platen gantry34. In some embodiments, platen 32 may engage and support a buildsubstrate 44, which may be a tray substrate as disclosed in Dunn et al.,U.S. Pat. No. 7,127,309, fabricated from plastic, corrugated cardboard,or other suitable material, and may also include a flexible polymericfilm or liner, painter's tape, polyimide tape (e.g., under the trademarkKAPTON from E.I. du Pont de Nemours and Company, Wilmington, Del.), orother disposable fabrication for adhering deposited material onto theplaten 32 or onto the build substrate 44. Platen gantry 34 is a gantryassembly configured to move platen 32 along (or substantially along) thevertical z-axis and is powered by z-axis motor 40.

Head carriage 36 is a unit configured to receive one or more removableprint heads, such as print heads 18, and is supported by head gantry 38.Examples of suitable devices for head carriage 36, and techniques forretaining print heads 18 in head carriage 36, include those disclosed inSwanson et al., U.S. Publication Nos. 2010/0283172 and 2012/0164256. Asmentioned above, in the second embodiment, each print head 18 may retaina velocimetry assembly, or head carriage 36 itself may retain a pair ofvelocimetry assemblies for use with each inserted print head 18.

As mentioned above, in some embodiments, guide tube 16 and/or print head18 may be components of system 10, rather than subcomponents ofconsumable assemblies 12. In these embodiments, additional examples ofsuitable devices for print heads 18, and the connections between printheads 18, head carriage 36, and head gantry 38 include those disclosedin Crump et al., U.S. Pat. No. 5,503,785; Swanson et al., U.S. Pat. No.6,004,124;LaBossiere, et al., U.S. Pat. Nos. 7,384,255 and 7,604,470;Batchelder et al., U.S. Pat. No. 7,896,209; and Comb et al., U.S. Pat.No. 8,153,182.

In the shown embodiment, head gantry 38 is a belt-driven gantry assemblyconfigured to move head carriage 36 (and the retained print heads 18) in(or substantially in) a horizontal x-y plane above chamber 30, and ispowered by x-y motors 42. Examples of suitable gantry assemblies forhead gantry 38 include those disclosed in Comb et al., U.S. Pat. No.13/242,561.

In an alternative embodiment, platen 32 may be configured to move in thehorizontal x-y plane within chamber 30, and head carriage 36 (and printheads 18) may be configured to move along the z-axis. Other similararrangements may also be used such that one or both of platen 32 andprint heads 18 are moveable relative to each other. Platen 32 and headcarriage 36 (and print heads 18) may also be oriented along differentaxes. For example, platen 32 may be oriented vertically and print heads18 may print 3D part 22 and support structure 24 along the x-axis or they-axis.

System 10 may also include a pair of sensor assemblies (not shown)configured to read encoded markings from successive segments of theconsumable filaments moving through guide tubes 16, such as disclosed inBatchelder et al., U.S. Patent Application Publication Nos.2011/0117268, 2011/0121476, and 2011/0233804; and in Swanson et al.,U.S. Publication Nos. 2010/0283172 and 2012/0164256. These sensorassemblies may be used in combination with the one or more velocimetryassemblies for providing one or more feed-forward and feedback controlloops to compensate for filament feed variations and/or extrudate flowvariations.

System 10 also includes controller assembly 46, which may include one ormore control circuits and/or one or more host computers configured tomonitor and operate the components of system 10. For example, one ormore of the control functions performed by controller assembly 46, suchas performing move compiler functions, can be implemented in hardware,software, firmware, and the like, or a combination thereof; and mayinclude computer-based hardware, such as data storage devices,processors, memory modules, and the like, which may be external and/orinternal to system 10.

Controller assembly 46 may communicate over communication line 48 withprint heads 18, chamber 30 (e.g., with a heating unit for chamber 30),head carriage 36, motors 40 and 42, sensor assemblies 44, the one ormore velocimetry assemblies, and various sensors, calibration devices,display devices, and/or user input devices. In some embodiments,controller assembly 46 may also communicate with one or more of bays 28,platen 32, platen gantry 34, head gantry 38, and any other suitablecomponent of system 10. While illustrated as a single signal line,communication line 48 may include one or more electrical, optical,and/or wireless signal lines, which may be external and/or internal tosystem 10, allowing controller assembly 46 to communicate with variouscomponents of system 10.

During operation, controller assembly 46 may direct z-axis motor 40 andplaten gantry 34 to move platen 32 to a predetermined height withinchamber 30. Controller assembly 46 may then direct motors 42 and headgantry 38 to move head carriage 36 (and the retained print heads 18)around in the horizontal x-y plane above chamber 30. Controller assembly46 may also direct print heads 18 to selectively draw successivesegments of the consumable filaments from container portions 14 andthrough guide tubes 16, respectively.

FIGS. 2A and 2B illustrate an example print head 18, which includeshousing 50 (having housing components 52 a and 52 b), motor assembly 54,and pump assembly 56 having drive mechanism 58 and liquefier assembly60, where liquefier assembly 60 includes extrusion nozzle 62. Examplesof suitable components for housing 50, motor assembly 54, and pumpassembly 56 include those discussed in Swanson et al., U.S. PublicationNo. 2012/0164256, Koop et al., U.S. patent application Ser. No.13/708,116; and Leavitt, U.S. patent application Ser. No. 13/708,037.

At each print head 18, controller assembly 46 directs motor assembly 54to transfer rotational power to drive mechanism 58 to feed successivesegments of the consumable filament to liquefier assembly 60. Liquefierassembly 60 thermally melts the received successive segments such thatthe consumable filament becomes a molten material. The molten materialresiding in nozzle 62 is then extruded from nozzle 62 and deposited ontoplaten 32 for printing 3D part 20 and support structure 22 in alayer-by-layer manner. After the print operation is complete, theresulting 3D part 20 and support structure 22 may be removed fromchamber 30, and support structure 22 may be removed from 3D part 20. 3Dpart 20 may then undergo one or more additional post-processing steps.

FIG. 3 further illustrates pump assembly 56 in use with guide tube 16,filament 64 (shown extending through cut-away portion of guide tube 16),extrudate velocimetry assembly 66, and filament-feed velocimetryassembly 68. As shown, filament drive mechanism 58 of pump assembly 56is located upstream from liquefier assembly 60, and is configured tofeed successive portions of filament 64 from guide tube 16 to liquefierassembly 60 based on the rotational power of motor assembly 54 (shown inFIG. 2B). As used herein, the terms “upstream” and “downstream” are madewith reference to a filament feed direction, such as along arrow 70, forexample.

Extrudate velocimetry assembly 66 is configured to measure the flow rateof the molten consumable material of filament 64 that exits nozzle 62,referred to as extrudate 72. As mentioned above, velocimetry assembly 66may be secured to print head 18, to head carriage 36, to purge station24, or any other suitable location of system 10. Velocimetry assembly 66preferably receives electrical power from system 10, and may communicatewith controller assembly 46 over communication line 48.

Additionally, filament-feed velocimetry assembly 68 may optionally beincluded to measure the feed rate of filament 64 prior to enteringliquefier assembly 60. In this case, velocimetry assembly 68 may bepositioned at any suitable location between and including container 14and liquefier assembly 60. For example, velocimetry assembly 68 may beretained by head carriage 36 to measure the feed rate of filament 64 inguide tube 16 (e.g., via an opening in guide tube 16) just prior toentering print head 18. Alternatively, velocimetry assembly 68 may beretained within housing 50 of print head 18 to measure the feed rate offilament 64 just prior to filament 64 entering liquefier assembly 60.

In a further alternative embodiment, velocimetry assembly 68 may bepositioned at any suitable location along the pathway of guide tube 16,such as at the one or more above-discussed sensor assemblies (not shown)to measure the feed rate of filament 68 in guide tube 16. In a furtheralternative embodiment, velocimetry assembly 68 may be retained in bay28 or in container 14 of a consumable assembly 12. When residing incontainer 14, the given container 14 may be configured to receiveelectrical power from system 10 and to communicate with controllerassembly 46 when loaded to bay 28.

As can be appreciated, when utilizing both velocimetry assemblies 66 and68, controller assembly 46 may compare the flow rates of extrudate 72 tothe feed rates of filament 64 to improve printing efficiencies and toidentify any anomalies that could indicate printing impairment (e.g.,filament slippage and material accumulation in liquefier assembly 60).Alternatively (or additionally), controller assembly 46 may monitordrive power applied to motor assembly 54 and/or any rotary encoderassociated with motor assembly 54 to detect potential filament feedanomalies that could indicate printing impairment.

As shown in FIG. 4, liquefier assembly 60 extends along longitudinalaxis 74, and includes liquefier tube 76. Liquefier tube 76 is a rigidtube fabricated from one or more thermally-conductive materials (e.g.,stainless steel), and includes outer surface 78 and inner surface 80, aswell as inlet end 82, outlet end 84, and channel 86 extendingtherebetween along longitudinal 74. Liquefier tube 76 is preferably isthin walled, having a wall thickness between outer surface 78 and innersurface 80 ranging from about 0.01 inches to about 0.03 inches, and morepreferably from about 0.015 to about 0.020. Preferred inner diametersfor liquefier 76 range from about 0.08 inches to about 0.10 inches, morepreferably from about 0.090 inches to about 0.095 inches.

The discussion of liquefier tube 76 is made herein with reference tolongitudinal axis 74 and a cylindrical geometry extending alonglongitudinal axis 74. However, in alternative embodiments, liquefiertube 76 may have a non-cylindrical geometry, such as disclosed inBatchelder et al., U.S. Patent Application Publication No. 2011/0074065.Accordingly, as used herein unless otherwise indicated, the term “tube”may include a variety of hollow geometries, such as cylindricalgeometries, elliptical geometries, polygonal geometries (e.g.,rectangular and square geometries), axially-tapered geometries, and thelike.

Liquefier assembly 76 is shown in use with thermal sleeve 88 (shown withhidden lines), which is an example heating element extending around adownstream segment of liquefier tube 76 to generate a hot zone alongliquefier tube 76 during a printing operation. Examples of suitableassemblies for thermal sleeve 88 include those disclosed in Swanson etal., U.S. Publication Nos. 2012/0018924 and 2012/0070523. Other suitableconfigurations for thermal sleeve 88 include a heating block asdisclosed in Swanson et al., U.S. Pat. No. 6,004,124, or other heatingelement configured to heat the hot zone. The thermal sleeve 88 may alsoinclude multiple heat-controlled zones to provide a multi-zone liquefiersuch as is disclosed in Swanson et al., U.S. Publication No.2012/0018924.

Nozzle 62 is a small-diameter nozzle secured to liquefier tube 76 atoutlet end 84, and is configured to extrude molten material at a desiredroad width. Preferred inner tip diameters for nozzle 62 includediameters up to about 760 micrometers (about 0.030 inches), and morepreferably range from about 125 micrometers (about 0.005 inches) toabout 510 micrometers (about 0.020 inches). In some embodiments, nozzle62 may include one or more recessed grooves to produce roads havingdifferent road widths, as disclosed in Swanson et al., U.S. patentapplication Ser. No. 13/587,002.

As further discussed in Swanson et al., U.S. patent application Ser. No.13/587,002, nozzle 62 may have an axial channel any suitablelength-to-diameter ratio. For example, in some embodiments, nozzle 62may have an axial channel with a length-to-diameter ratio to generatehigh flow resistance, such as a ratio of about 2:1 to about 5:1. Inother embodiments, nozzle 62 may have an axial channel with alength-to-diameter ratio to generate lower flow resistance, such as aratio less than about 1:1. Accordingly, suitable length-to-diameterratios for the axial channel of nozzle 62 may range from about 1:2 toabout 5:1, where in some low-flow resistance embodiments, ratios rangingfrom about 1:2 to about 1:1 may be preferred for use with the method ofthe present disclosure.

Suitable consumable filaments for filament 64 include those disclosedand listed in Crump et al., U.S. Pat. No. 5,503,785; Lombardi et al.,U.S. Pat. Nos. 6,070,107 and 6,228,923; Priedeman et al., U.S. Pat. No.6,790,403; Comb et al., U.S. Pat. No. 7,122,246; Batchelder, U.S. PatentApplication Publication No. 2009/0263582; Hopkins et al., U.S. PatentApplication Publication No. 2010/0096072; Batchelder et al., U.S. PatentApplication Publication No. 2011/0076496; and Batchelder et al., U.S.Patent Application Publication No. 2011/0076495.

Furthermore, filament 64 may include encoded markings, as disclosed inBatchelder et al., U.S. Patent Application Publication Nos.2011/0117268, 2011/0121476, and 2011/0233804, which may be used with theabove-discussed sensor assemblies of system 10; and/or topographicalsurfaces patterns (e.g., tracks) as disclosed in Batchelder et al., U.S.Pat. No. 8,236,227. The length of filament 64 may be any suitablelength, and is preferably more than about 100 feet.

Additionally, filament 64 may have a non-cylindrical geometry, such as aribbon filament as disclosed in Batchelder et al., U.S. Pat. No.8,221,669. In this embodiment, print head 18 may include a ribbonliquefier assembly as disclosed in Batchelder et al., U.S. ApplicationPublication No. 2011/0074065; and in Swanson et al., U.S. ApplicationPublication No. 2012/0070523, and as briefly mentioned above.

During the printing operation, drive mechanism 58 (shown in FIG. 2B)feeds filament 64 into channel 86 of liquefier tube 76 from inlet end82, in the direction of arrow 70. Thermal sleeve 88 heats the encasedregion of liquefier tube 76 to one or more elevated temperatures togenerate hot zone 90. The heating of liquefier tube 76 at hot zone 90melts the material of filament 64 to form melt 92.

The molten portion of the filament material (i.e., melt 92) formsmeniscus 94 around the unmelted portion of filament 64. During anextrusion of melt 92 through nozzle 62, the downward movement offilament 64 in the direction of arrow 70 functions as a viscosity pumpto extrude the material in melt 92 out of nozzle 62 as extrudate 72 forprinting 3D part 20 or support structure 22.

Changes in the flow rate of extrudate 72, such as when starting,stopping, accelerating, and decelerating is controlled by changing thefeed rate of filament 64 with drive mechanism 58 (shown in FIG. 2B),based on drive commands from controller assembly 46. However, the flowrate of extrudate 72 out of nozzle 62 does not always respond the sameto changes in the feed rate of filament 64. For example, extrudate 72may flow at different rates from nozzle 62 for the same feed rate offilament 64 into liquefier tube 76. This is due to numerous non-steadystate conditions within liquefier tube 64, such as changes in the meltflow characteristics of the consumable material, previous changes infilament feed rates and extrudate flow rates (e.g., during previousstarting, stopping, accelerating, and/or decelerating), response timedelays, and the like.

In an open loop design, without any feedback measurements of extrudate72, controller assembly 46 typically operates print heads 18 based onpredictive models on how extrudate 72 will flow. However, due to thevirtually unlimited 3D part geometries that can be printed with system10, it can be difficult to predict how print heads 18 will function inevery situation. Furthermore, open loop designs will not detect gradualchanges in print head 18 over time, such as liquefier scaling, materialaccumulation, and the like.

Instead, the extrudate flow rates measured and determined by velocimetryassembly 66 can be compared to filament drive commands, allowing system10 to predict how extrudate flow rates will change based on a variety ofoperation conditions (i.e., calibrate print heads 18). Additionally,when velocimetry assembly 66 is retained by print head 18 or headcarriage 36, the determined extrudate flow rates may be determined whileperforming printing operations. Each of the techniques can assist inreducing response time delays, and improving part quality and printingrates.

FIG. 5 illustrates an example embodiment for velocimetry assembly 66 inuse with a print head 18 for measuring flow rates of extrudate 72. Dueto its molten state, extrudate 72 has a generally smooth cylindricalsurface of the molten part or support material, which provides verylittle particulate structure for optically measuring a flow rate.However, extrudate 72 exhibits light scattering, rendering it suitablefor use with velocimetry assembly 66.

In the shown embodiment, velocimetry assembly 66 includes laser source96, optionally one or more optic lenses 98 (illustrated as a collectionlens 98), and detector 100, where detector 100 preferably communicateswith controller assembly 46 over communication line 48. Laser source 96is an example light source for velocimetry assembly 66, and may be anysuitable laser beam-generating device for generating and directing apulsed laser beam 102 towards collection lens 98, such as a laser diodeor even an Nd:YAG laser, which may be bandwidth filtered to isolate anysuitable wavelength.

Examples of suitable wavelengths “λ,” for laser beam 102 range fromabout 300 nanometers to about 900 nanometers. In some embodiments, thewavelength “λ,” for laser beam 102 ranges from about 600 nanometers toabout 900 nanometers. In other embodiments, the wavelength “λ,” forlaser beam 102 ranges from about 400 nanometers to about 600 nanometers.In alternative embodiments, velocimetry assembly 66 may utilizedifferent types of light sources for emitting light beams, such assuper-radiant light-emitting diodes (LEDs) and the like.

Optic lenses 98 may include any suitable lens or set of lenses foroptically expanding, condensing, collimating, and/or routing laser beam102 toward a segment of extrudate 72 (i.e., at location below nozzle 62along longitudinal axis 74), where the illuminated segment of extrudate72 has a length “L” along longitudinal axis 74. Upon reaching extrudate72, a portion of the light from laser beam 102 is scattered back towardssensor 104 as scattered rays 104, where the pattern of scatter rays 104is based on a speckle pattern of extrudate 72 at the particularilluminated segment.

The dimensions for length “L” will vary depending on the focal volume oflaser beam 102 relative to extrudate 72. Depending on the compositionand color of the consumable material for extrudate 72, extrudate 72 maybe partially transmissive such that the focal volume of laser beam 102may penetrate into extrudate 72 to illuminate the core region ofextrudate 72, for example. This will affect the dimensions for length“L”, as well as the scatter pattern of rays 104.

Accordingly, due to the reflective nature of scatter rays 104 in thisembodiment (in comparison to the transmission-forward-scatter patterndiscussed below for velocimetry assemblies 134, 138, and 152, shownrespectively in FIGS. 10-12), laser beam 102 may have a focal volume atabout the exterior surface of extrudate 72, referred to as extrudatesurface 72 a. This allows velocimetry assembly 66 to determine thevelocity of extrudate 72 at the extrudate surface 72 a.

However, extrudate surface 72 a may flow at a slower rate than its coreregion, providing a velocity gradient along its radius. This radialvelocity gradient is typically greatest at nozzle 62, and equalizes inthe extrudate after traveling a short distance from nozzle 62. As such,in this embodiment, the focal volume of laser beam 102 is preferablymaintained at a point that is sufficiently distant from nozzle 62 suchthat the radial velocity gradient substantially equalizes betweenextrudate surface 72 a and the core region of extrudate 72. Theresulting velocity determined by velocimetry assembly 66 may thencorrespond to an average velocity with a low standard deviation over theradius of extrudate 72.

Velocimetry assembly 66 may also optionally include one or moreadditional optics (not shown) for expanding, condensing, collimating,and/or routing scattered rays 104 toward detector 100, wherein detector100 has a sensor 106 with a plurality of photodetector pixels located ata distance “D” from the surface of extrudate 72 in a direction along thex-axis (i.e., perpendicular to longitudinal axis 74).

Detector 100 may be any suitable sensor device for detecting theintensities of scattered rays 104 in at least a one-dimensional (1D)pattern (i.e., in an array of pixels 106 extending along the z-axis),and or in a (2D) two-dimensional pattern (i.e., in a matrix of pixels106 oriented in the y-z plane). As discussed below, in some embodiments,detector 100 may include multiple sensors 106, such as multiple 1Dsensors, which can also be used to measure the width of extrudate 72 fordetermining the volumetric flow rate of extrudate 72, as discussedbelow.

Briefly, upon exiting nozzle 62, extrudate 72 also typically exhibitsdie swelling that can vary the diameter of extrudate by as much as 35%.As such, by measuring the diameter of extrudate 72 at the segment thatis illuminated by laser beam 102, the cross-sectional area of theilluminated segment of extrudate 72 can be measured. Combining themeasured cross-sectional area and the measured velocity may then providevolumetric flow rate for the illuminated segment of extrudate 72.

Examples of suitable devices for detector 100 include digital cameras,such as high-speed cameras incorporating charge-coupled device (CCD) orcomplementary metal-oxide-semiconductor (CMOS) chips. The intensity oflaser beam 102 is preferably high enough such that the integration timeof detector 100 is short enough to capture and process successive imagesof scattered rays 104 at a frame rate “R” that is synchronized in timingwith the pulsing of laser beam 102. Accordingly, detector 100 preferablyincludes a timing circuit that captures frames at sensor 106 in responseto each pulse of laser beam 102.

As illustrated in FIG. 6, when successive frames of scattered rays 104are captured by sensor 106, detector 100 (and/or controller assembly 46)may process the signals to generate a speckle pattern for each frame,such as the speckle pattern 108 plotted based on signal intensity “I”for successive frames 110 a, 110 b, and 110 c. As shown, the speckles ofextrudate 72 have an average speckle spacing “S”, may be determined byEquation 1:

$\begin{matrix}{S = \frac{\lambda*D}{L}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where “λ,” is the wavelength of laser beam 102, “D” is the distancebetween pixels 106 and the surface of extrudate 72 in a direction alongthe x-axis (i.e., perpendicular to longitudinal axis 74) as definedabove, and “L” is a spot length of the segment of extrudate 72 alonglongitudinal axis 74 that is illuminated by laser beam 102 as definedabove.

Examples of suitable distances “D” range from about 0.1 inches to about5 inches, more preferably from about 0.1 inches to about 3 inches.Similarly, examples of suitable spot lengths “L” range from about 5 milsto about 100 mils, more preferably from about 10 mils to about 60 mils.

The speckle structure is determined by the details of index variationsand surface features in extrudate 72. As such, apart from relativisticeffects and mode structure within laser beam 102, speckle pattern 108captured on detector 100 moves at the same velocity as extrudate 72along longitudinal axis 74. This is illustrated in FIG. 6 with thedownward spatial shift in speckle pattern 108 between the successiveframes 110 a, 110 b, and 110 c. Therefore, the downward (or positive)velocity of extrudate 72 along longitudinal axis 74 in the direction ofarrow 70 may be determined as a function of the spatial shift of specklepattern 108 between the successive frames 110 a, 110 b, and 110 c.Similarly, if extrudate 72 is drawn back into nozzle 62 during aroll-back step, the upward (or negative) velocity of extrudate 72 alonglongitudinal axis 74 in an opposite direction from arrow 70 may also bedetermined as a function of the spatial shift of speckle pattern 108between successive frames.

Detector 100 and/or controller assembly 46 may also calculate thevolumetric flow rate of extrudate 72 based on the predicted averagecross-sectional area of extrudate 72 and the determined velocity.Alternatively, as discussed below, detector 100 may also include asecond sensor (not shown in FIG. 6) for measuring the width of extrudate72 at each frame. In this embodiment, the actual volumetric flow ratemay then be calculated based on the measured extrudate width and thedetermined velocity.

FIGS. 7-9 illustrate an example technique and algorithm for determiningthe spatial shift of speckle pattern 108 between successive frames(e.g., frames 110 a, 110 b, and 110 c). For example, a mis-match betweensuccessive frames may be determined by Equation 2:

$\begin{matrix}{{{Err}(i)} = {\frac{1}{W - i}{\sum_{c}\left( {{I\left( {f,r,{c + i}} \right)} - {I\left( {{f + 1},r,c} \right)}} \right)^{2}}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

where I(f, r, c) is the video or signal intensity of the pixel 106 fromframe “f”, row “r” and column “c”; “i” is the number of pixels 106 thateach successive frame is delayed by along longitudinal axis 74; and “W”is the total number of pixels in sensor 106 along longitudinal axis 74that are used to capture the frames.

FIGS. 7 and 8 correspondingly illustrate example logic diagrams 112 and114 for comparing speckle patterns 108 of successive frames. Forexample, as shown by logic diagram 112 in FIG. 7, one of the rows ofpixels 106 along longitudinal axis 74 may capture a frame in a dual portrandom access memory (RAM) module. Furthermore, a line from a previousframe may be read out by a complimentary clock, so that one dual portRAM module can store both a new line and recall the previous line.

As discussed above, velocimetry assembly 66 measures extrudate flowrates by comparing speckle data over two or more successive measurements(i.e., over two or more captured frames). Due to this requiredchronological comparison, a speckle pattern 108 in a single frame itselfdoes not provide enough information to measure the extrudate flow rate.As such, as shown by logic diagram 114 in FIG. 8, if the velocity ofextrudate 72 is positive (i.e., moving along longitudinal axis 74 in thedirection of arrow 70), the live image needs to be delayed.

Alternatively, if the velocity of extrudate 72 is negative, (i.e.,moving along longitudinal axis 74 opposite of the direction of arrow 70)the previous frame image needs to be delayed. The amount of delay varieswith the velocity of extrudate 72, and can be up to one-half of thenumber of pixels in sensor 106 along longitudinal axis 74 that are usedto capture the frames (i.e., W/2).

As further shown in FIG. 8, the sum of the squares of the differencesbetween the two line scans may be computed for multiple different delayssimultaneously (e.g., seventeen different delays), both because thedelay may not be an estimated delay, and because the rate of change ofthe error near the best fit delay may allow extrapolation of a moreprecise velocity of extrudate 72. Velocimetry assembly 66 (or controllerassembly 46) may then conduct a search for the delay with the best fitor lowest error delay, which may then be used and updated in subsequentframes. In alternative embodiments, velocimetry assembly 66 may use anysuitable logic design to perform this function, such as with combfilters. When all seventeen errors have been measured, the smallest set(e.g., smallest three) may be used in an algorithm, such as depicted bymethod 116 in FIG. 9, which may be performed by controller assembly 46to determine the measured velocity and the estimated delay for the nextframe.

As shown in FIG. 9, method 116 includes steps 118-132 for determiningthe measured velocity of extrudate 72 and the estimated delay for thenext frame captured by detector 100. As mentioned above, the flow rateof extrudate 72 out of nozzle 62 does not always respond the same tochanges in the feed rate of filament 64. For example, extrudate 72 mayflow at different rates from nozzle 62 for the same feed rate offilament 64 into liquefier tube 76. This is due to numerous non-steadystate conditions within liquefier tube 64, such as changes in the meltflow characteristics of the consumable material, previous changes infilament feed rates and extrudate flow rates (e.g., during previousstarting, stopping, accelerating, and/or decelerating), response timedelays, and the like.

To account for these numerous non-steady state conditions, method 116performs a best fit analysis to identify how the velocity of extrudate46 responds to different operating conditions, as shown in steps 120,122, 124, 126, and 130, for example. If the best fit delay is zero (step128), the direction may be changed for the next frame, which switcheswhether the live or stored data is delayed with respect to the other.

Furthermore, if the best fit delay is zero for both the positivedirection (i.e., in the direction of arrow 70) and the negativedirection (i.e., opposite of the direction of arrow 70), the storedframe is not updated until the best fit delay is non-zero. This changesthe conversion of pixel delay to velocity, but allows greater precisionfor near-zero velocities of extrudate 72. For the extrapolationcalculation to be valid, there are preferably multiple pixels perspeckle spacing S (e.g., seventeen pixels per speckle spacing S).

Furthermore, extrudate 72 has a maximum velocity and a maximumacceleration that can be monitored and processed by detector 100. Forexample, the maximum velocity “ν_(max)” of the speckles that can bemonitored by a detector 100 with a frame rate of “R” (in hertz) may bedetermined by Equation 3:

$\begin{matrix}{v_{{ma}\; x} = \frac{\lambda \; {DR}}{16p}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

where “λ,” is the wavelength of laser beam 102 as defined above, “D” isthe distance between pixels 106 and the surface of extrudate 72 in adirection along the x-axis (i.e., perpendicular to longitudinal axis 74)as defined above, and “p” is the average size of each pixel in sensor106 along longitudinal axis 74.

The maximum velocity of extrudate 72 may depend on a variety of factors,including the drive pressure in liquefier tube 76, the composition ofextrudate 72, the extent of melting of extrudate 72, the dimensions ofnozzle 62, and the like. However, a suitable volumetric flow rate forextrudate 72 having an 18-mil average diameter may be about 300micro-cubic inches/second (mics), which corresponds to an velocity ofabout 1.2 inches/second. For a distance “D” between the pixels of sensor106 and the surface of extrudate 72 of about two inches, a pixel size“p” of about 4 micrometers, a laser spot size “L” of about 40 mils, anda laser beam wavelength of about 650 nanometers, this only requires aframe rate of about 60 hertz, which is readily attainable with a varietyof commercially available CMOS image sensors.

The maximum acceleration (or deceleration) of extrudate 72 is a measureof how far off the estimated velocity can be and still find the minimumerror delay. With the above-discussed algorithm for method 116, theestimated delay can be in error by seventeen pixels at most, providing amaximum acceleration “A_(max)” for the above-discussed embodiment asdetermined by Equation 4:

A_(max)=17 p R²  (Equation 4)

Following the previous example, extrudate 72 may decelerate from avolumetric flow rate of about 300 mics to zero mics at an initial rateof about 3,000 mics/second, corresponding to an acceleration of about0.03 g-forces. For a pixel size “p” of about 4 micrometers, thisacceleration/deceleration also only requires a frame rate of about 60hertz.

If greater velocities are desired, such as for faster printing rates,any easy adjustment to make is to select a detector 100 having a fasterframe rate. Accordingly, suitable frame rates for detector 100 includeframe rates of at least about 50 hertz, more preferably at least about60 hertz, and even more preferably at least about 100 hertz. In someembodiments, a higher-end CMOS or CCD chip may be utilized having veryfast frame rates, such as frame rates greater than about 1 kilohertz,more preferably greater than 10 kilohertz, and even more preferablygreater than 20 kilohertz.

For example, for a distance “D” is the distance between sensor 106 andthe surface of extrudate 72 of about two inches, a sensor pixel array of2,080 pixels each with a pixel size “p” of about 4 micrometers, a laserspot size “L” of about 20 mils, a laser beam peak wavelength of about640 nanometers, and a frame rate of about 28 kilohertz, about 50 toabout 100 speckles may be detected, which provides a good compromisebetween flow rate and detector sensitivity.

The above-discussed velocimetry assembly 66 is suitable for determiningvelocities for extrudate 72 after exiting nozzle 62. Signals relating tothe measured velocities may be relayed to controller assembly 46 viacommunication line 48, which may then perform one or more functionsbased on the received signals, as discussed above. For example, in thefirst embodiment, velocimetry assembly 66 may be used as a calibrationtool for each print head 18 (e.g., at a purge station 24 or a separate,stand-alone station). Alternatively, in the second embodiment,velocimetry assembly 66 may determine extrudate flow rates whileprinting 3D part 20 and support structure 22. Each of these uses canassist in reducing response time delays, and in improving part qualityand printing rates.

Additionally, filament-feed velocimetry assembly 68 (shown in FIG. 3)may be used in the same manner as velocimetry assembly 66 to measurefeed rates of filament 68 fed to print head 18 via drive mechanism 58(or other filament drive mechanism). This may assist in further reducingresponse times, and may also detect potential filament feed issues, suchas filament breakage, jamming, and the like. Additionally, controllerassembly 46 may compare the flow and feed rates from velocimetryassemblies 66 and 68 to perform additional functions for system 10, suchas for detecting filament slippage at drive mechanism 58.

FIG. 10 illustrates a second embodied extrudate velocimetry assembly 134for use with system 10 as a replacement for velocimetry assembly 66. Asshown, velocimetry assembly 134 may function in a similar manner tovelocimetry assembly 66. However, in this case, detector 100 is locatedat an opposing side of extrudate 72 from laser source 96. This takesadvantage of the scattering of the light rays of laser beam 102, whichalso exhibit a transmission-forward-scatter pattern, as shown.

This configuration is preferred as it provides the highest intensitylight, which correspondingly reduces the integration time for detector100. In fact, the laser power required for the configuration can bereduced by about ten times compared to the reflection-based scatterconfiguration discussed above for velocimetry assembly 66. This reducedintegration time can increase the frame rate of detector 100, which, asdiscussed above, increases the maximum velocity andacceleration/deceleration that detector 100 can monitor.

As shown in FIG. 10, velocimetry assembly 134 may also include one ormore optic lenses 136 (a single lens 135 is shown), which may includeany suitable lens or set of lenses for optically expanding, condensing,collimating, and/or routing scattered rays 104 toward detector 100.Detector 100 may operate in the same manner as discussed above invelocimetry assembly 66, but with the increased usable frame rate duethe shorter integration times.

As mentioned above, depending on the composition and color of theconsumable material for extrudate 72, extrudate 72 may be partiallytransmissive such that the focal volume of laser beam 102 may penetrateinto extrudate 72 to illuminate the core region of extrudate 72, forexample. This will affect the dimensions for length “L”, as well as thescatter pattern of rays 104.

In this embodiment utilizing a transmission-forward-scatter pattern, thefocal volume of laser beam 102 may be located either at core region orat extrudate surface 72 a. If located at extrudate surface 72 a, thefocal volume of laser beam 102 is preferably maintained at a point thatis sufficiently distant from nozzle 62 such that the radial velocitygradient substantially equalizes between extrudate surface 72 a and thecore region of extrudate 72. As discussed above, the resulting velocitydetermined by velocimetry assembly 66 may then correspond to an averagevelocity with a low standard deviation over the radius of extrudate 72.

Measuring the velocity of extrudate 72 at a point that is further awayfrom nozzle 62 may also be beneficial for reducing the risk of havingwater vapor interfere with the scattering of laser beam 102, which canotherwise produce a false speckle velocity. Additionally, the furtherdistance reduces the risk of sensing light that scatters from nozzle 62.

Examples of suitable distances along longitudinal axis 74 from nozzle 62for positioning the center of the focal volume of laser beam 102,referred to as distance “F”, for this application include distances ofat least about 10 mils, more preferably from about 15 mils to about 50mils, and even more preferably from about 20 mils to about 30 mils.Suitable distances “L” for use with these distances “F” include thosediscussed above, and may even more preferably range from about 10 milsto about 30 mils. These ranges have been found to be suitable formultiple tip pipe inner diameters for nozzle 62, such as tip pipe innerdiameters of about six mils to about twelve mils, for example.

However, in some situations, it may be preferred to measure extrudate ata point that is closer to nozzle 62, where extrudate surface 72 a andthe core region flow at different rates. For example, the closerlocation to nozzle 62 reduces the amount that extrudate 72 may wanderfrom the axis of nozzle 62. Additionally, the closer segments ofextrudate 72 also typically exhibit less stretching by the weight ofpreviously-extruded materials, and may exhibit less distortion from acircular cross-section. Accordingly, in these situations, the focalvolume of laser beam 102 may be located at the core region of extrudate72. The resulting velocity determined by velocimetry assembly 66 maythen correspond to the velocity of the core region of extrudate 72.

FIG. 11 illustrates a third embodied extrudate velocimetry assembly 138,which may function in a similar manner to velocimetry assembly 134(shown in FIG. 10). However, in this case, velocimetry assembly 138 alsoincludes beam splitter 140 and mirror 142, and detector 100 alsoincludes a second sensor 144 of photodetector pixels. In thisembodiment, sensors 106 and 144 may each include a single array ofpixels extending along the longitudinal axis 74 (i.e., 1D sensors).Sensor 106 may function in the same manner as discussed above todetermine the velocity of extrudate 72.

In comparison, sensor 144 receives the split rays 146 from scatteredrays 104, which are routed from beam splitter 140 and mirror 142 to thepixels of sensor 144. Sensor 144 is also preferably synchronized intiming with the frame captures of sensor 106 and the pulsing of laserbeam 102. This allows sensor 144 to measure the widths of successivesegments of extrudate 72 that are illuminated by the focal volume oflaser beam 102. These measured widths may be converted tocross-sectional areas, which, when combined with the determinedvelocities, allows detector 100 (and/or controller assembly 46) todetermine the volumetric flow rates of extrudate 72 (rather than relyingon a predicted cross-sectional area, as discussed above).

In this embodiment, the focal volume of laser beam 102 is preferablypositioned at the extrudate surface 72 a, and maintained at a point thatis sufficiently distant from nozzle 62 such that the radial velocitygradient substantially equalizes between extrudate surface 72 a and thecore region of extrudate 72. As discussed above, the resulting velocitydetermined by velocimetry assembly 66 may then correspond to an averagevelocity with a low standard deviation over the radius of extrudate 72.This also allows the split rays 146 to account for the entire width ofextrudate 72 when received by sensor 144.

As discussed above, upon exiting nozzle 62, extrudate 72 typicallyexhibits die swelling that can vary the diameter of extrudate by as muchas 35%. As such, by measuring the width (i.e., diameter) of extrudate 72at the segment that is illuminated by laser beam 102, thecross-sectional area of the illuminated segment of extrudate 72 can bemeasured. Combining the measured cross-sectional area and the measuredvelocity may then provide volumetric flow rate for the illuminatedsegment of extrudate 72.

One limitation of velocimetry assemblies 66, 134, and 138 is that theyeach require a line-of-sight to extrudate 72 to direct laser beam 102and to receive the scattered rays 104. In some extrusion-based additivemanufacturing systems, nozzle 62 may positioned close the underlyinglayers of 3D part 20 and support structure 22. This could potentiallyprevent velocimetry assemblies 66, 134, and 138 from reaching extrudate72 upon exiting nozzle 62.

Instead, in an alternative embodiment, the velocimetry assembly may beintegrated into the nozzle to monitor the velocity of pre-extrudateprior to exiting nozzle 62. For example, as shown in FIG. 12, nozzle 148may be secured to outlet end 84 of liquefier tube 76 in ascrew-engagement manner (or with any other suitable engagement) with awasher 150 (e.g., an indium washer) that functions as a pressure seal,where nozzle 148 may be fabricated from a transparent material, such asa glass.

Velocimetry assembly 152 may accordingly include laser source 96, inletlight pipe 154, outlet light pipe 156, and detector 100, where inletlight pipe 154 and outlet light pipe 156 may be coupled to an externalcylindrical surface 158 of nozzle 148, allowing laser beam 102 toilluminate pre-extrudate 160 in a tip pipe 162 of nozzle 148. Examplesof suitable dimensions for tip pipe 162 include those discussed abovefor nozzle 62.

The couplings between light pipes 154 and 156 and surface 158 of nozzle148 preferably allow laser beam 102 and scattered rays 104 to transmitthrough the surface coupling without substantially scattering or unduetransmission loss. In some embodiments, the couplings may fixedly securelight pipes 154 and 156 to surface 158. Alternatively, light pipes 154and 156 may be integrally fabricated with nozzle 148 as a single piece.In a further alternative, and more preferred embodiment, light pipes 154and 156 may be coupled to surface 158 in a manner that allows lightpipes 154 and 156 to mechanically separate from nozzle 148. Thisembodiment is beneficial when nozzle 148 needs to be replaced, allowingthe same liquefier tube 76 and light pipes 154 and 156 to be used withmultiple, interchangeable nozzles 148.

As shown, laser source 96 generates laser beam 102, which is routedthrough inlet light pipe 154 and nozzle 148 toward tip pipe 162. At tippipe 162, laser beam 102 contacts pre-extrudate 160 prior, whichscatters the light of laser beam 102 with the sametransmission-forward-scatter pattern as discussed above for velocimetryassemblies 134 and 138 (shown in FIGS. 10 and 11). The resultingscattered rays 104 may then transmit through nozzle 148 and into outletlight pipe 156, where they may be collected and routed to detector 100for processing, as discussed above.

The distance “D” between the pixels of sensor 106 and the focal volumeof laser beam 102 in or at pre-extrudate 160 in tip pipe 162 is theaverage distance that the routed light rays 102 travel through nozzle148 and outlet light pipe 156, which, in this case, does not follow aliner path. Preferably, in this embodiment, the focal volume of laserbeam 102 is positioned at the core region of pre-extrudate 72 to reducethe extent that shear effects of the inner surface of tip pipe 62 at thesurface of pre-extrudate 160 affect the determined velocity.

This technique has the advantage that pre-extrudate 160 in tip pipe 162may be maintained at a near constant temperature, which prevents changesin speckling and allows the molten material to retain a substantiallyconstant level of optical transmission. Furthermore, since thedimensions of tip pipe 162 are constant (assuming no scaling or materialaccumulation), the cross-sectional area of pre-extrudate 160 is known,allowing the volumetric flow rate of pre-extrudate 160 (and therefore,of extrudate 72) to be readily calculated from its determined velocity.

Velocimetry assembly 152 illustrates an example nozzle and velocimetryassembly of system 10 and/or print head 18 that may be used to determinevelocities and volumetric flow rates in a quasi-real time manner duringprinting operations to print 3D part 20 and support structure 22. Basedon the determined velocities and/or volumetric flow rates, controllerassembly 46 may perform one or more functions to compensate for flowvariations. Furthermore, velocimetry assembly 152 also be used as acalibration tool, as discussed above, and/or for any other suitable usein system 10 (e.g., detecting tip clogging). Each of these uses canassist in reducing response time delays, and in improving part qualityand printing rates with system 10.

Although the present disclosure has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the disclosure.

1. An additive manufacturing system comprising: a print head forprinting a three-dimensional part in a layer-by-layer manner using anadditive manufacturing technique, wherein the print head is configuredto receive a consumable material, melt the consumable material, andextrude the molten material; a velocimetry assembly configured todetermine flow rates of the molten material; and a controller assemblyconfigured to manage the extrusion of the molten material from the printhead, and to receive signals from the velocimetry assembly relating tothe determined flow rates.
 2. The additive manufacturing system of claim1, wherein the velocimetry assembly comprises: a light source, andoptionally, one or more optical lenses, which are configured to route alight beam from the light source towards the molten material, whereinthe molten material causes light rays of the light beam to scatter; adetector having a sensor configured to receive at least a portion of thescattered light rays, and to transmit the signals relating to thedetermined flow rates to the controller assembly.
 3. The additivemanufacturing system of claim 2, wherein the detector is positionedrelative to the light source to receive the scattered light rays in atransmission-forward-scatter pattern.
 4. The additive manufacturingsystem of claim 2, wherein the detector is configured to receive theportion of the scattered light rays over multiple captured frames,wherein each captured frame has a speckle pattern, and wherein thevelocimetry assembly is configured to compare the speckle pattern of themultiple captured frames to determine velocities of the molten material.5. The additive manufacturing system of claim 4, wherein the detector ofthe velocimetry assembly further comprises a second sensor configured toreceive at least a portion of the scattered light rays, wherein thedetector is further configured to determine the flow rates as volumetricflow rates of the molten material from the scattered light rays receivedby the second sensor and the determined velocities of the moltenmaterial.
 6. The additive manufacturing system of claim 1, wherein thevelocimetry assembly is configured to determine velocities of the moltenmaterial after the molten material exits a nozzle of the print head asan extrudate.
 7. The additive manufacturing system of claim 1, andfurther comprising a purge station, wherein the velocimetry assembly isretained by the purge station.
 8. The additive manufacturing system ofclaim 1, wherein the velocimetry assembly is configured to route apulsed laser beam into an extrusion nozzle of the print head.
 9. Amethod for using an additive manufacturing system, the methodcomprising: providing a print head retained by the additivemanufacturing system; feeding a consumable material to the print head;melting the consumable material in the print head to produce apre-extrudate of the molten consumable material; extruding thepre-extrudate from the print head as an extrudate; and determining flowrates of the pre-extrudate or the extrudate with a velocimetry assembly.10. The method of claim 9, and further comprising determining velocitiesof the consumable material being fed to the print head with a secondvelocimetry assembly.
 11. The method of claim 9, wherein determining theflow rates of the pre-extrudate or the extrudate with the velocimetryassembly comprises: routing a pulsed light beam toward the pre-extrudateor extrudate to scatter light rays from the pre-extrudate or extrudate;and detecting at least a portion of the scattered light rays with afirst detector over multiple captured frames that are synchronized intime with a pulsing of the light beam.
 12. The method of claim 11,wherein the first detector is positioned to receive the scattered lightrays in a transmission-forward-scatter pattern.
 13. The method of claim11, and further comprising: detecting at least a portion of thescattered light rays with a second detector over multiple capturedframes that are synchronized in time with the detecting at the firstdetector; determining cross-sectional areas of the extrudate orpre-extrudate based on the scattered light rays detected with the seconddetector; and determining the flow rates as volumetric flow rates fromthe determined velocities and the determined cross-sectional areas. 14.The method of claim 9, wherein the extrudate or pre-extrudate comprisesthe extrudate, wherein the pulsed light beam comprises a pulsed laserbeam, and wherein routing the pulsed laser beam toward the extrudatecomprises positioning a focal volume of the pulsed laser beam at aposition at which velocities of a core region and a surface of theextrudate are substantially the same.
 15. A method for using an additivemanufacturing system, the method comprising: extruding a molten materialfrom a print head retained by the additive manufacturing system as anextrudate; routing a pulsed laser beam toward the extrudate to scatterlight rays of the pulsed laser beam from the extrudate; detecting atleast a portion of the scattered light rays with a detector overmultiple captured frames, wherein each captured frame has a specklepattern; and comparing the speckle pattern of the multiple capturedframes to determine flow rates of the extrudate.
 16. The method of claim15, wherein routing the pulsed laser beam toward the extrudate comprisespositioning a focal volume of the pulsed laser beam at a position atwhich velocities of a core region and a surface of the extrudate aresubstantially the same.
 17. The method of claim 15, and furthercomprising: detecting at least a portion of the scattered light rayswith a second detector over multiple captured frames; determiningcross-sectional areas of the extrudate or pre-extrudate based on thescattered light rays detected with the second detector; and determiningthe flow rates as volumetric flow rates from the determined velocitiesand the determined cross-sectional areas.
 18. The method of claim 15,wherein the detector is retained at a purge station of the additivemanufacturing system.
 19. The method of claim 9, wherein the detector ispositioned to receive the scattered light rays in atransmission-forward-scatter pattern.
 20. The method of claim 15,wherein the detecting step is performed while the print head is printinglayers of a three-dimensional part using an additive manufacturingtechnique.